1. Field of the Invention
The present invention relates to an electronic device fabrication apparatus which is specifically suitable as a plasma chemical vapor deposition apparatus usable for forming a semiconductor thin film formed of amorphous silicon hydride and an insulative film, or suitable as a plasma etching apparatus usable for processing a semiconductor device, a liquid crystal device and the like. In this specification, plasma chemical vapor deposition will be referred to as "plasma CVD", and amorphous silicon hydride will be referred to as "a-Si:H".
2. Description of the Related Art
Today, electronic device fabrication apparatuses, such as, for example, plasma CVD apparatuses and plasma etching apparatuses are in wide use for fabricating metal films, semiconductor films, dielectric films and crystal-line wafers.
An exciting frequency of a high frequency power supply for generating plasma used in most of such electronic device fabrication apparatuses is of a radiowave (frequency: around 13.56 MHz; also referred to as "RF" or high frequency "HF") or of a microwave (frequency: around 2.45 GHz; also referred to as "MW").
Recent active studies in plasma physics have found theoretically, as well as based on experiments, that an intermediate frequency band between the radioactive wave frequency band and the microwave frequency band is suitable for use in fabrication of electronic devices. The intermediate frequency band includes a band of around 100 MHz which is referred to as the "VHF" (very high frequency) band and a band around several hundred mega-hertz which is referred to as the "UHF" (ultra high frequency) band.
A. A. Howling, et al., "J. Vac. Sci. Technol. A10 (1992), page 1080", for example, describes dependency of SiH.sub.4 plasma on frequency in the case where the VHF band is used for an exciting frequency. T. Kitamura et al., "Plasma Sources Sci. Technol. 2 (1993), pp. 40-45" and "S. Oda, Plasma Sources Sci. Technol. 2 (1993), pp. 26-29", for example, describe effects of using the VHF band for an exciting frequency.
Japanese Laid-Open Publication No. 6-77144 describes a usable high frequency range, and Japanese Laid-Open Publication No. 6-5522 describes the use of high frequency power which is permitted by altering the surface structure of a cathode electrode. Japanese Laid-Open Publication No. 7-273038 describes insertion of an insulative pipe in an intermediate portion of a gas introduction pipe which is used for introducing gas into a reaction chamber. The above-mentioned publication also describes a technology regarding the position and the inner diameter of the insulative pipe.
FIG. 14 is a schematic view of a conventional plasma CVD apparatus 700. The conventional plasma CVD apparatus 700 includes a reaction chamber 5. The reaction chamber 5 accommodates a cathode electrode 2 and an anode electrode 4 which are opposed to each other. The anode electrode 4 has a substrate 6 provided on a surface thereof which faces the cathode electrode 2. The reaction chamber 5 further accommodates a heater 7 provided on a rear wall 5b thereof.
Reaction gas 8 is introduced into the reaction chamber 5 through a gas introduction pipe 1. One end 1a of the gas introduction pipe 1 is connected to the cathode electrode 2. The reaction gas 8 is blown into the reaction chamber 5 through a gas ejection hole (not shown) formed in the cathode electrode 2. The gas introduction pipe 1 has an insulative pipe 10 in an intermediate portion thereof. A high frequency power generation device 3, including a high frequency power supply 3a and a matching circuit 3b, is provided in the vicinity of the end 1a of the gas introduction pipe 1.
Use of an intermediate region between the RF and the MW as an exciting frequency of the high frequency power supply 3a is advantageous for (1) increasing the plasma density in proportion to the square of the frequency and (2) realizing such a high plasma density by a relatively low potential.
Advantage (1) above refers to the fact that the film deposition rate increases in proportion to the square of the frequency in the case where the intermediate region is used for film deposition, and refers to the fact that the etching rate increases in proportion to the square of the frequency in the case where the intermediate region is used for etching. Accordingly, the fabrication efficiency of electronic devices is enhanced.
Advantage (2) above restricts plasma damage, i.e., damage on the film or substrate caused by ions contained in the plasma even during the high rate film formation or etching.
FIG. 15 is a graph illustrating impedances in the reaction chamber 5 and the gas introduction pipe 1 with respect to the frequency in the plasma CVD apparatus 700. When only a voltage having a frequency in the RF band is used for generating plasma, impedance .vertline.Z.vertline. between the cathode electrode 2 and the anode electrode 4 in the reaction chamber 5 is about 10 to 20 .OMEGA., whereas impedance .vertline.Z.vertline. in the gas introduction pipe 1 is as high as about 300 .OMEGA.. Accordingly, very little high frequency power is supplied to the gas introduction pipe 1, which permits electric discharge to occur in the reaction chamber 5.
In an electronic industry which is referred to as giant microelectronics, such as solar cells and liquid crystal display devices using thin films formed of a-Si:H-type materials, substrates have a relatively great length of, for example, about 40 to 60 cm. In order to realize a high throughput, a reaction chamber for processing a plurality of such large substrates at one time is required. For a semiconductor device fabrication apparatus, for example, it is very important that a plurality of substrates should be processed at one time in order to realize a high throughput.
For the above-described reasons, a reaction chamber unavoidably has a side of as great as about 1 meter. More specifically, surfaces of the cathode electrode and the anode electrode which face each other, i.e., electrode surfaces need to be increased. The size of the electrode surfaces is in the same order as that of the wavelength of the VHF band.
Therefore, in order to realize a high throughput, it is necessary to use a frequency in the VHF or UHF band, which corresponds to the size of the fabrication apparatus (i.e., size of the reaction space between the cathode electrode and the anode electrode) as an exciting frequency supplied from the high frequency power supply. With the prior art described in Japanese Laid-Open Publication No. 6-77144, 6-5522 and 7-273038, the following problems (1) and (2) occur. Due to such problems, utilization of plasma generated in the reaction space over a large surface area is prevented.
(1) The increase in size of the cathode electrode and the anode electrode increases the floating capacitance, which raises the impedance between the cathode electrode and the anode electrode, i.e., the impedance in the reaction chamber. PA1 (2) In the case when the high frequency power is absorbed in the insulative pipe 10 in the gas introduction pipe 1 as described above, electrolytic dissociation of the reaction gas can disadvantageously occur; i.e., the reaction gas is disadvantageously converted into a plasma state. PA1 .pi.: ratio of the circumference of the circle to the diameter, PA1 f: exciting frequency, PA1 L.sub.F : inductance at a position which is electrically connected in series to a portion having the same potential as that of the cathode electrode for a DC voltage, PA1 C.sub.F : capacitance between a portion having the same potential as that of the cathode electrode for a DC voltage and a portion having a ground potential, and PA1 L: inductance at a position which is electrically connected in series to the insulative pipe in the gas introduction pipe. PA1 .pi.: ratio of the circumference of the circle to the diameter, PA1 f: exciting frequency, PA1 L.sub.F : inductance at a position which is electrically connected in series to a portion having the same potential as that of the cathode electrode for a DC voltage, PA1 C.sub.F : capacitance between a portion having the same potential as that of the cathode electrode for a DC voltage and a portion having a ground potential, and PA1 C: capacitance between the insulative pipe and the portion having the ground potential. PA1 .pi.: ratio of the circumference of the circle to the diameter, PA1 f: exciting frequency, PA1 L.sub.F : inductance at a position which is electrically connected in series to a portion having the same potential as that of the cathode electrode for a DC voltage, PA1 C.sub.F : capacitance between a portion having the same potential as that of the cathode electrode for a DC voltage and a portion having a ground potential, PA1 L: inductance at a position which is electrically connected in series to the insulative pipe in the gas introduction pipe, and PA1 C: capacitance between the insulative pipe and the portion having the ground potential. PA1 .pi.: ratio of the circumference of the circle to the diameter, PA1 f: exciting frequency, PA1 L.sub.F : inductance at a position which is electrically connected in series to a portion having the same potential as that of the cathode electrode for a DC voltage, PA1 C.sub.F : capacitance between a portion having the same potential as that of the cathode electrode for a DC voltage and a portion having a ground potential, and PA1 C: capacitance between the insulative pipe and the portion having the ground potential. PA1 .pi.: ratio of the circumference of the circle to the diameter, PA1 f: exciting frequency, PA1 L.sub.F : inductance at a position which is electrically connected in series to a portion having the same potential as that of the cathode electrode for a DC voltage, PA1 C.sub.F : capacitance between a portion having the same potential as that of the cathode electrode for a DC voltage and a portion having a ground potential, and PA1 L: inductance of a portion of the gas introduction pipe other than the coil, the portion being provided at a position connected to the coil having an inductance L.sub.4 in series between a position having the same potential as that of the cathode electrode for a DC voltage and the position having the ground potential.
Especially in the plasma CVD apparatus 700 (FIG. 14) described in Japanese Laid-Open Publication No. 7-273038, in which the insulative pipe 10 is provided in an intermediate portion of the gas introduction pipe 1, when high frequency power in the VHF band is supplied, the insulative pipe 10 has a capacitance. The capacitance causes resonance, and as a result, impedance .vertline.Z.vertline. in the gas introduction pipe 1 becomes smaller than impedance .vertline.Z.vertline. in the reaction chamber 5.
As a consequence, the high frequency power is absorbed by the insulative pipe 10 in the gas introduction pipe 1, and the insulative pipe 10 is heated excessively. Therefore, the amount of power introduced to the reaction chamber 5 is reduced, thus preventing electric discharge between the cathode electrode 2 and the anode electrode 4.
Moreover, such a phenomenon can disadvantageously melt and destroy the insulative pipe 10 and thus cause safety problems.
When gas used for film formation or dry etching is electrolytically dissociated before being introduced to the reaction chamber, the film formation or etching is not performed. Furthermore, generation of plasma is not performed in the reaction chamber. For example, when SiH.sub.4 gas, which is used for forming a silicon film in the reaction chamber, is electrolytically dissociated before being introduced into the reaction chamber, silicon loses its reaction activity and enters the reaction chamber in a microscopic powder state. Silicon in such a state causes, for example, deterioration in film quality and particle contamination.
Accordingly, the gas introduction pipe is preferably structured to prevent the electrolytic dissociation.